The invention is related to the field of thermal technology, and more specifically to cooling computing systems.
Electronic components from microprocessors to high-end power converters generate heat. The rejection of this heat is necessary for their optimum and reliable operation. As electronic design allows higher throughput in smaller packages, dissipating the heat load becomes a critical design factor.
There are different ways of dissipating heat. These include radiation, conduction and convection. Radiation means that the heat is simply radiated away from the object, through electromagnetic radiation. Conduction is the exchange of kinetic energy between molecules. Less energetic (lower temperature) particles gain kinetic energy by colliding with more energetic particles (through physical contact).
Convection is heat transfer by movement of a heated substance (gas or liquid). This means that the heat is transferred to the molecules of the gas (or liquid) surrounding the hot object, and then transported away through movement of molecules. When the gas or liquid around the object is forced into movement (e.g., by a fan blowing air across a heat sink), then it is forced convection. Many of today""s electronic devices require cooling beyond the capability of standard metallic heat sinks.
A heat pipe is a type of cooling device where heat is decapitated by forced convection. A heat pipe is essentially a passive heat transfer device with an extremely high effective thermal conductivity. The heat pipe in its simplest configuration is a closed, evacuated cylindrical aluminum or copper vessel with internal walls lined with a capillary structure or wick that is saturated with a working fluid. The working fluid enters the pores of the wicking material, wetting all internal surfaces. Since the heat pipe is evacuated and then charged with the working fluid prior to being sealed, the internal pressure is set by the vapor pressure of the fluid.
As heat enters the heat pipe at an evaporator end of the heat pipe, the heat causes the working fluid to vaporize. The vaporized fluid creates a pressure gradient, which forces the vapor to flow along the pipe to a cooler section (a condenser end of the heat pipe) where it condenses giving up its latent heat of vaporization. The working fluid is then returned to the evaporator end of the heat pipe by capillary forces developed in the wick structure. The wicking material serves as a pump to return the cooled working fluid from the condenser end.
Heat pipes can be designed to operate over a very broad range of temperature. In electronic cooling applications where it is desirable to maintain junction temperatures below 125-150 degrees Celsius, copper/water heat pipes are typically used. There are many factors to consider when designing heat pipe including the amount of power the heat pipe is capable of transferring and the effective heat pipe thermal resistance.
Heat pipes can be built in different sizes, shapes and materials to accommodate different application geometries. Using heat pipes is advantageous in space-constrained systems. For example, in mobile computing systems, there isn""t enough space to put a heat exchanger on top of a microprocessor to dissipate heat. The heat pipe transfers the heat with minimal drop in temperature and allows the heat exchanger to be placed elsewhere in the mobile computing system.
Current applications of the heat pipes include cooling Pentium processors in notebook computers, especially in notebook computers, due to the limited space and power. However, the application of the heat pipe in the notebook computers is limited to using a single large heat pipe. The large heat pipe is connected to a thermal attach block which is connected to a die. However, the single large heat pipe does not provide efficient cooling. In addition, the single large heat pipe is not flexible enough to accommodate different space and size constraints.